1. Field of the Invention
The present invention relates a depositing method and a surface modifying method for nano-particles in a gas stream and, more in particular, it relates to a depositing method and a surface modifying method for nano-particles in a gas stream capable of efficiently depositing nano-particles with a particle diameter of less than 100 nm for which development of various functions not obtainable in their bulk state can be expected based on the quantum size effect under a charged state in a gas streams, and capable of the modifying the surface of the nano-particles which are extremely sensitive to defects or impurities caused by a large exposure ratio of surface atoms at a good controllability in the gas stream.
2. Related Art
When a material is reduced into the size of nano-particles having a particle diameter of less than 100 nm, it sometimes develops physical property and function not shown in the bulk state thereof. Referring, for example, to the light emitting function, it has been known that even group IV elements such as silicon (Si) and germanium (Ge) which are indirect transition semiconductor and scarcely emit light in their bulk state emit intense visible lights at a room temperature when they are reduced into nano-particles of several nm diameter. However, since the ratio of atoms exposed on the surface is extremely large in nano-particles (reaching 40% at the grain diameter of 5 nm), the nano-particles are extremely sensitive to the intrusion of impurities or presence of crystal defects and it has been so far considered difficult for the development of them into high functional devices.
For example, in a case of Si nano-particles, it is described that Si nano-particles formed in an atmospheric gas show a quantum size effect as the optical characteristic which is not obtainable in their bulk state as shown in the literature (xe2x80x9cStructures and Optical Properties of Silicon Nanocrystals Prepared by Pulsed-Laser Ablation in Inert Background Gasxe2x80x9d, Appl. Phys. Lett. Vol. 76 pp 1389-1391, 2000).
When nano-particles in a state charged to single polarity are deposited, deposits are charged as the amount of deposition increases, particularly, in semiconductors or insulator materials, in which repulsion exerts between nano-particles and deposits, and the amount of deposition is sometimes saturated when deposition is conducted to a predetermined amount or more.
Further, in a case of using nano-particles as they are or by assembling nano-particles for functional materials or functional devices, it is necessary to suppress coalescence and agglomeration of nano-particles and arranging them irregularly or regularly in a predetermined pattern on a substrate in order to possess the functionality inherent to the size. However, since the nano-particles of the size concerned in this invention has high surface activity and remarkable lattice-softening or melting effect on the surface, it has been difficult by the existent method for suppression of coalescence and agglomeration, or regular or irregular arrangement. In a case where nano-particles or an assembly of nano-particles as functional materials or functional devices, it is necessary for treatments to further emphasize the characteristic of the nano-particles or provide different kinds of functions. However, the treatment mentioned above tends to deteriorate the characteristic inherent to the nano-particles.
This invention provides a method of suppressing saturation for the deposition amount of nano-particles by providing a step of neutralizing static charges for nano-particles charged to a single polarity in a gas stream before a collecting/deposition step, as well as modifying the surface modification of nano-particles by providing a step of mixing a first gas species for carrying nano-particles and a second gas species for conducting surface modification and a step of activating a gas mixture of the first gas species and the second gas species and nano-particles carried on the gas mixture before the collecting/deposition step.
In accordance with the method described above, charged nano-particles in the gas stream can be deposited efficiently, and the surface of the nano-particles extremely sensitive to defects or impurities caused by the large exposure ratio of the surface atoms can be modified in the gas stream at a good controllability.
The method of depositing the nano-particles according to the invention has a feature comprising a step of neutralizing charges of the nano-particles charged to the single polarity in the gas stream before the collection/deposition step. This provides an effect of efficient deposition while suppressing saturation of the deposition amount of the charged nano-particles in the gas stream.
This invention further provides a particle deposition method which includes neutralizing charges by the irradiation of ultraviolet rays in vacuum, or neutralizing charges by the irradiation of electron beams in a step of neutralizing the charges of the nano-particles upon deposition of the nano-particles. In accordance with method, high vacuum can be kept in the collection/deposition step to improve the cleanness in the process, as well as can increase the inertial force of the particles in the gas stream to locally conduct centralized deposition.
Further, this invention provides a surface modifying method for particles including a step of mixing a first gas species for carrying the nano-particles and a second gas species for modifying the surface and a step of irradiating ultraviolet rays in vacuum for a gas mixture of the first gas species and the second species and nano-particles carried on the gas mixture before the collection/deposition step. This method provides an effect of activating the second gas species thereby causing them to react with the nano-particles to modify the surface of the nano-particles.
Further, this invention provides a surface modifying method for particles in which the ultraviolet rays for activating the second gas species is an excimer laser light. By the use of the excimer laser light of favorable shaping property, the second gas species can be activated at a good controllability.
Furthermore, this invention provides a surface modifying method for particles including a step of mixing a first gas species for carrying nano-particles and a second gas species for modifying the surface, and a step of irradiating an electron beam to a gas mixture of a first gas species and a second gas species and nano-particles carried on a gas mixture before a collection/deposition step. This method can maintain high vacuum in the collection/deposition step to improve the cleanness of the process, as well as can increase the inertial force of the surface modified nano-particles to locally conduct centralized deposition.
Furthermore, this invention provides a surface modifying method for particles including a step of mixing a first gas species carrying nano-particles and second gas species applying surface modification and a step of radiation-heating by infrared rays a gas mixture of a first gas species and a second gas species and nano-particles carried on a gas mixture before a collection/deposition step. This method can control the thickness of a modified portion formed on the surface of the nano-particles by applying heat treatment to the nano-particles and the second gas species simultaneously.
As has been described above, according to this invention, charged nano-particles in the gas stream can be deposited efficiently, as well as nano-particles extremely sensitive to defects or impurities caused by large exposure ratio of surface atoms can be surface modified in the gas stream at a good controllability.
Accordingly, this invention intends at first to provide an excellent method of depositing nano-particles in a gas stream, as well as a surface modifying method capable of efficiently depositing nano-particles with the particle diameter of less than 100 nm for which development of various functions not obtainable in their bulk due to the quantum size effect, in a charged state in a gas stream regarding deposition and surface modification of nano-particles in the gas stream.
This invention further intends to provide an excellent deposition method and a surface modifying method for nano-particles in a gas stream capable of applying surface modification to nano-particles sensitive to defects or impurities caused by large exposure ratio of surface atoms in a gas stream at a good controllability.